This study aimed to determine longitudinal associations of early triadic family processes and 3-year-old children’s strengths and difficulties and to control those associations for family risk factors. In 80 families expecting their first child, we assessed parents’ anticipations of future family relationships (Triadic Capacity) and parents’ psychological distress, marital quality, and education level. When the children were 4 months of age, we observed triadic family interaction in a standardized laboratory play scenario. The children’s strengths and difficulties at age three were assessed using multiple methods. As expected, parents’ Triadic Capacity assessed before the child was born predicted triadic family interaction 4 months after birth. Early triadic family processes explained variance in children’s emotional functioning at age three over and above the effects of family stress factors assessed before the child was born. However, early triadic family processes did not explain children’s co-operative behaviour or children’s symptoms at age three. Results also highlighted the roles of fathers’ education level in children’s externalizing behaviour, mothers’ psychological distress at children’s low co-operative behaviour, and low marital quality in children’s internalizing behaviour.